Of note, and to further complicate the confusion, seaborne services of the United States and many other nations refer to the officer in charge of any seagoing vessel as "captain" regardless of actual rank. For instance, in the civilian merchant marine, the commander of a vessel is also referred to as a captain, and everyone in his boat must abide by his commands. While not an official rank, as in military pay grade, it is an official title and can confuse many individuals who are new to or unfamiliar with naval traditions. One exception to the rule is when an officer of higher rank than captain, such as admiral, is the officer in charge of one or more seagoing vessel(s), such as a carrier battle group. An admiral is never referred to as "captain". An additional item of note is that in the seaborne services, especially for submarine and aviation commands, the commanding officer is often referred to informally as "skipper" whether the officer is a captain or below.

For the ground and air forces rank, a captain is of pay grade O-3 (the third officer rank), usually serving as the commander of a company-sized unit in the ground forces, as a flight leader or other squadron officer in air units, or serving as an executive officer or staff officer for a larger unit such as a battalion or squadron. This rank is used by the U.S. Army, U.S. Air Force, and U.S. Marine Corps.

The rank of captain is also used in other organizations outside of the military, particularly in fire departments, police, and law enforcement.

The insignia for captains in the United States Army was first authorized in 1836. During the American Civil War, the rank of Captain was used in both the Union Army and Confederate States Army. The rank was typically held by either junior staff officers or company commanders. In the case of the latter, company officers were normally elected by the men of their unit, unless the officer in question held rank in the Regular Army. In cases where regiments had suffered high casualties, it was not uncommon for a captain to assume duties as the regimental commander. Such was the case in the 1st Mississippi Infantry which was commanded by Captain Owen Hughes after the regiment had lost over half its number at the Battle of Nashville.

Due to the ambiguity created by the common use of "captain" for officers of different grades between the Army, Navy, Air Force, Marine Corps, and Coast Guard, equivalency is conferred between officers by use of identical pay grade rather than title of rank. The higher the grade, the higher the rank of the officer. For example, an Army, Air Force, or Marine captain is equivalent in rank, responsibilities, and grade to a Navy or Coast Guard lieutenant, all of which are of the third officer grade, or O-3. Similarly, a Navy or Coast Guard captain is equivalent in rank, responsibilities, and grade to an Army, Air Force, or Marine colonel, all of which are of the sixth officer grade, or O-6. Thus, Army, Air Force, and Marine colonels together with Navy and Coast Guard captains wear the silver eagle insignia of the grade of O-6, while Army, Air Force, and Marine captains together with Navy and Coast Guard lieutenants wear the double silver bars of the rank insignia of the grade of O-3. Additionally, the O-6 rank of Navy and Coast Guard captain is abbreviated as uppercase "CAPT", while the O-3 rank of captain is abbreviated as "CPT" for the Army and mixed-case "Capt" for the Air Force and Marine Corps.

In the United States, the rank of captain first appeared in the Continental Army during the Revolutionary War. A captain was the officer placed in charge of a company of soldiers and was granted a commission from the regimental colonel. A captain was afforded one to several lieutenants, depending on the size of the company, and the captain’s commission could be revoked or expired at the end of a particular military campaign.

The Continental Navy used the rank of captain as the commander of a sailing vessel at war, with the captain having several lieutenants on board, as well as a Sailing Master to assist in their duties. This use of the rank carried over into the U.S. Navy. With the addition of the ranks of commander and lieutenant commander between lieutenant and captain, a Navy captain became equivalent in rank to an Army colonel.

[1] No universal insignia for officer candidate rank; Navy candidate insignia shown[2]Official 1945 proposal for General of the Armies insignia; John J. Pershing's GAS insignia: ; George Dewey's Admiral of the Navy insignia: [3] Rank used for specific officers in wartime only, not permanent addition to rank structure[4] Grade is authorized by the U.S. Code for use but has not been created[5] Grade has never been created or authorized[6] USAF and U.S. Army insignia shown